The invention relates to a system for controlling the speed of a motor of an electric fan unit. The field of application is notably that of heating and/or air conditioning installations, in particular for automobiles.
For such applications, a known solution is to produce an electric fan unit of the type comprising a volute casing, a turbine mounted inside the volute casing for generating an air flow within the latter, a turbine drive motor, and a control module for the motor comprising a control system that allows the speed of the motor to be varied according to the requirements.
Usually, the speed control system for the motor of an electric fan unit comprises an field-effect transistor of the MOS type operating in pulse-width modulation (PWM) mode.
However, pulse-width modulation generates abrupt variations in voltage and current causing electromagnetic interference commonly called EMC.
One problem to be solved is therefore the reduction of this electromagnetic interference.
For this purpose, it is known that this reduction depends, amongst other things, on the slope of the current as the field-effect transistor switches.
FIGS. 5A and 5B show two conventional examples of speed control systems for a motor 101 comprising a field-effect transistor 103.
FIG. 5A shows that a circuit 105, comprising in series a source of voltage 107 of value V and a resistive element 109 of resistance Rg, is connected between the gate G and the source S of the field-effect transistor 103.
A power supply source 115 is connected to the drain D of the field-effect transistor 103 and to a terminal of the motor 101. The other terminal of the motor 101 is connected to the source S of the field-effect transistor 103. In addition, a flywheel circuit 113 is connected across the terminals of the motor 101.
In this case, the switching current I through the field-effect transistor 103 depends on the resistance Rg and on an intrinsic capacitance Cgs of this field-effect transistor 103, according to the following formula:
      I    =                  V                  Rg          ×          Cgs                    ×      t        ,where t is time.
Thus, the slope P of the current is given by the following formula:
      P    ⁡          (              Rg        ,        Cgs            )        =      V          Rg      ×      Cgs      
Consequently, an increase in the value of the resistance Rg reduces the slope P of the current and also that of the voltage as the field-effect transistor switches.
However, the reduction in the voltage and current slopes generates thermal losses that could diminish the reliability and the performance of the field-effect transistor 103.
Moreover, the intrinsic capacitance Cgs varies from one component to another owing to the manufacturing tolerances of these components. This capacitance variability from one transistor to another causes differences in the level of electromagnetic interference from one product to another. This is unacceptable for an automobile manufacturer.
In order to reduce the effect of the intrinsic capacitance, a usual solution consists in adding a capacitor.
Indeed, FIG. 5B shows a control system similar to that in FIG. 5A but where a capacitor 117 of capacitance Cgk is added between the gate G and the source S of the field-effect transistor 103.
Thus, according to the control system in FIG. 5B, the slope P of the current as the field-effect transistor 103 switches is given by the following formula:
      P    ⁡          (              Rg        ,        Cgs        ,        Cgk            )        =      k          Rg      ⁡              (                  Cgs          +          Cgk                )            
According to this formula, the reduction in the slope of the current is controlled by an increase in the capacitance Cgk.
However, an increase in the capacitance Cgk generates a power dissipation in the resistance Rg. Thus, a large value of capacitance Cgk could degrade or even destroy the resistive element 109, thus limiting the performance of the capacitance Cgk for controlling the slope of the current. The size of the couple formed by the capacitance Cgk and resistive element 109 is therefore a limiting factor.
Furthermore, the variation in the intrinsic capacitance Cgs from one component to another always leads to a significant variability in the current slope, which in turn causes a large dispersion in the level of electromagnetic interference from one product to another.